a=h+ 1.2

b=h+0.2

(a+b)h/2=0.78

A= 1.8 b=0.8 is obtained by solving the above three equations simultaneously.

2 root number (81.31%) = 90.17% This is the answer.

3 According to the problem, the equation (x-3200) [20+(4600-x)/100 *10] = 64000.

The solution is x=4000, so the charging standard is set at 4000.